calculo de ct

Nov-06

© Siemens AG 2006Power Transmission and DistributionPTD EA13/Funk

CT – Dimensioning

Guideline

Busbar protection 7SS52 / 7SS60

Nov-06


Equivalent scheme of the current transformer

Rprim Xprim Rsec Xsec

X

Nov-06


Simplified scheme of the current transformer

Rprim Xprim Rsec Xsec

X Zburden

Nov-06


Burden of a CT (simplified equivalent scheme)

Rct = internal burden

Rl = resistance current loop

(leads)

Rr = relay(s) burden

R´b = Rr + Rl

(connected burden)

Rct Rl

RrVknee Vterm

Nov-06


Current Transformer Identification (IEC 60044-6)

10 P 10; 15VA

Rated CT power

Rated short-circuit current factor Kssc

Core type: P = Protection M = Measurement

Error in % at Kssc x IN

Nov-06


Required data (IEC60044 terms)

System data: Iscc max maximum short-circuit current(…to be considered)

CT data: Sn rated CT powerRb rated resistive burdenKssc rated symmetrical short-circuit current

factor Rct secondary winding resistanceCT circuit data: R´b connected burden (Rleads + Rrelay)

Notes: CT data according to ANSI, IEEE, BS standard should be converted to

IEC Rct as a dominant parameter should be given or measured, the 20%Sn

assumption could lead to an overdimensioning of the CT with 5A CTs attention should be payed to the resistance of the leads

Nov-06


Ktd = 0,5 (7SS5, 7SS6) transient dimensioning factor

(=relay property)

*) maximum relay measuring range

Remark: Assumed stabilizing factor for calculation k=0.5 The default setting of k=0.65 ensures safety margin

Step 1: Requirement for symmetrical short-circuit current factor K´ssc

Iscc max IN

Ktd < K´ssc < 100 *

Required K´ssc

Nov-06


Step 2: Calculation of effective symm. short-circuit current factor K´ssc

K´ssc = effective symmetrical short-circuit current factor („KnALF“)

Kssc = rated symmetrical short-circuit current factor („KoALF“)

Rb = rated restive burden

Rct = secondary winding resistance

Rl = resistance of leads

RRelay = relay burden

R´b = Rl + Rrelay (connected burden)

Rb + Rct

K´ssc = Kssc R´b + Rct

Effective K´ssc

Nov-06


Step 3: Cross checking

If the effective K´ssc is higher than the required K´ssc: the CT is correctly dimensioned no further calculation or simulation is necessary the default setting of k=0.65 (7SS52), resp. K=0.6 (7SS60) is suitable

Otherwise please contact your Siemens consultantfor assistance (e.g. CT simulation)

Nov-06


Example: 1. required K´ssc

Iscc max = 30kA

89 BB600/1 Transformer: 600/1, 5P10, 15VA

Sec. winding resistance: 4

Leads: 50m, 4mm2 CU

Relay burden: 0,1

Iscc max 30kA =0,5 = 25 IN 600A

K´ssc > Ktd

Required K´ssc

Nov-06


Iscc max = 30kA

15 + 4K´ssc = 10 = 42

0,55 + 4

Required was: K´ssc > 25Requirement fulfilled, CT correctly dimensioned!

Example: 2. effective K´ssc

Transformer: 600/1, 5P10, 15VA

Sec. winding resistance: 4

Leads: 50m, 4mm2 CU

Relay burden: 0,1

89 BB600/1

l mm2 50mRl = 2 = 2•0.0179• A m 4mm2

Rb = Sn / Isn2

=0.45

Nov-06


Stability verification for bus protection

This calcution is a first and rough consideration

If the effective K´ssc is about the required K´ssc, a closer view to the

saturation free time (>3ms) and the stabilization factor k is necessary

simulation of the CT behaviour

or detailled calculation

Nov-06


CT-simulation (L1)

Nov-06


CT-simulation (L2)

Nov-06


CT-simulation (L3)

Nov-06


Burden of busbar protection 7SS52 and 7SS60

7SS52

Max. burden In=1A 0.1 VA / 0.1

In=5A 0.2 VA / 0.01

7SS60 (max. total burden of relay)

summation CT (4AM5120-3/4DA00-0AN2) 1.8 VA / 2.5 VA

matching CT (4AM5120-1/2DA00-0AN2) 1 VA / 1.2 VA

matching CT (4AM5272-2AA/3AA00-0AN2) 2 VA

Nov-06


Conversion of CTs: IEC 60044 to BS 3938

CT design according to BS 3938

The CT is defined by the knee-point voltage VK and the secondary winding resistance Rct. The design values according to IEC 60044 can be approximately transfered into the British Standard definition by following formula:

BS CT definition

Isn rated secondary current

Example: IEC 60044: 600/1, 5P10, 15 VA, Rct = 4

BS:

Vk = 146V ; Rct = 4

3.1

)( sscsnctbk

KIRRV

VVVk 1463.1

101)415(

Nov-06


Conversion of CTs: BS 3938 to IEC 60044

When a CT according to BS definition (VK, Rct, Isn) has to be transfered into IEC definition the two design values Kssc and Rb have to be defined. Therefor one parameter can be choosen, e.g. from a data sheet. The other one will be determined by the equation then.

ctssc

snknct

snssc

kb

snctb

kssc

SK

IVSR

IK

VR

IRR

VK

3.13.1

)(

3.1

Kssc Sn

10 15VA

20 5.5VA

30 2.3VA

Example:

Vk = 146V ; Rct = 4; Isn = 1A

Nov-06


Conversion of CTs: IEC 60044 to ANSI, IEEE C57.13

CT design according to ANSI/ IEEE C57.13

Class C of this standard defines the CT by its secondary terminal voltage Vst at20 times rated current, for which the ratio error shall not exceed 10%.Standard classes are C100, C200, C400 and C800 for 5 A rated secondary current. This terminal voltage can be approximately calculated from the IEC data as follows:

ANSI CT definition

20520max..

sscbts

KRAV

getweAIandI

SRwith sn

sn

nb 5

2

A

KSV sscn

ts 5max..

Nov-06


Conversion of CTs: ANSI, IEEE C57.13 to IEC 60044

When a CT according to ANSI definition (Cxxx) has to be transfered into IEC definition the equivalent burden has to be calculated.

Example: C200, 5A

burdennominalP class502)5(´

burdenstandardANSI2520

200

20´

22

max..

VAARIS

AI

VR

bnn

n

tsb

„ANSI“-CT In = 5A (typ.) In = 1A

C100 10P20, 25VA 10P20, 5VA

C200 10P20, 50VA 10P20, 10VA

C400 10P20, 100VA 10P20, 20VA

C800 10P20, 200VA 10P20, 40VA

calculo de ct

Documents

circuit current

iec definition

matching ct

ct data

current transformer

ieee c57

iec 60044when

effective